In the direct injection engine, since a time interval from a fuel injection until a fuel combustion is relatively short, it is necessary to increase a fuel injection pressure for atomizing the fuel. An electric low-pressure pump pumps up the fuel from a fuel tank. A mechanical high-pressure pump pressurizes the fuel and discharges the fuel toward the fuel injector.
Generally, in the direct injection engine, a fuel pressure sensor is provided to detect a fuel pressure which is supplied to the injector. A discharge rate of the high-pressure pump is feedback controlled in such a manner that the detected fuel pressure agrees with a target fuel pressure. The low-pressure pump is driven under a specified constant condition (constant driving voltage), and a pressure regulator adjusts the discharge pressure of the low-pressure pump.
The low-pressure pump is driven under the constant condition even if a fuel consumption is varied. Thus, in a case that the fuel consumption is low, a discharge rate of the low-pressure pump is excessive, which may waste a battery voltage to deteriorate the fuel economy.
In view of the above, it is required that the discharge rate of the low-pressure pump is made as low as possible to improve the fuel economy. However, if the discharge rate of the low-pressure pump is made low, the fuel pressure in a low-pressure fuel passage between the low-pressure pump and the high-pressure pump is also decreased. It is likely that the fuel is evaporated in the low pressure fuel passage to generate a vapor when the high-pressure pump suctions the fuel. Such a vapor may deteriorate a fuel discharge efficiency of the high-pressure pump, so that the discharge pressure of the high-pressure pump can not be brought to a target fuel pressure and a malfunction may be caused in the high-pressure pump.
A patent document 1 (JP-2003-222060A) shows a technology of preventing a generation of vapor, in which a temperature-pressure relation expression is previously established and a target pressure P0 is derived from the temperature-pressure relation expression. A fuel pressure P1 at which a vapor (cavitation) is actually generated in the high-pressure pump is obtained. Based on a difference between the target pressure P0 and the fuel pressure P1, the temperature-pressure expression is corrected.
Moreover, in a port injection engine equipped with a low-pressure fuel pump without a high-pressure pump, as shown in a patent document 2 (Japanese Patent No. 3060266: U.S. Pat. No. 5,483,940) and a patent document 3 (JP-2007-315378A: US-2007-0251501A1), a fuel pressure sensor is provided to detect a fuel pressure discharged from the fuel pump and the fuel pump is feedback controlled such that the detected fuel pressure agrees with the target fuel pressure.
However, in the technology shown in the patent document 1, it is necessary to actually generate a vapor in the high-pressure pump when the temperature-pressure expression is corrected. Thus, it is likely that a malfunction may be caused in the high-pressure pump by the vapor and a reliability of the fuel supply system may be deteriorated.
Further, it is conceivable that the technologies shown in the patent document 1 and the patent document 2 are applied to a fuel injection system having a low-pressure pump and a high-pressure pump. A fuel pressure sensor is provided for detecting a fuel pressure in a low-pressure fuel passage. The low-pressure pump is feedback controlled in such a manner that the detected fuel pressure agrees with a target fuel pressure to restrict a generation of vapor. However, in this case, both the fuel pressure sensor detecting low pressure fuel and the fuel pressure sensor detecting high fuel pressure are necessary, which increase a product cost of the fuel injection system.